CHEMISTRY: BAXTER, GROVER AND THORVALDSON 
71 
A REVISION OF THE ATOMIC WEIGHT OF LEAD: THE 
ANALYSIS OF LEAD BROMIDE AND CHLORIDE 
By Gregory Paul Baxter, Fred Leslie Grover and Thorbergur Thorvaldson 
COOUEXiE MEMORIAL LABORATORY, HARVARD UNIVERSITY 
Presented to the Academy, January 3, 1914 
In a recent paper by Baxter and Wilson^ the atomic weight of lead 
was found from analyses of fused lead chloride, by comparison with pure 
silver and by weighing the silver chloride produced. The result of this 
investigation, Pb = 207.09 (Ag = 107.880), is considerably higher than 
most earlier determinations. For this reason and because the work of 
Baxter and Wilson was essentially preliminary in its nature it was 
obviously desirable to test the matter further, and ever since the publi- 
cation of the above paper the atomic weight of lead has been under al- 
most continuous investigation in the Harvard Laboratory. Because of 
the advantages offered by the greater insolubility of silver bromide, the 
analysis of lead bromide was next undertaken; and later lead chloride 
was investigated anew. The outcome of all the work is that the atomic 
weight of lead is very close to 207.20, a value even higher than that pre- 
viously found by Baxter and Wilson. 
The method of experimentation was the usual one employed with 
metalHc haHdes. Carefully purified lead bromide, which had been 
crystallized from concentrated hydrobromic acid solution, was dried by 
fusion in an atmosphere containing a large proportion of hydrobromic 
acid gas. The acid was synthesized from hydrogen and bromine, and 
during a fusion was mixed with either nitrogen, hydrogen, or bromine. 
After being weighed the salt was dissolved in hot water containing a 
very small amount of nitric acid, and at low concentration the solution 
was precipitated with a very dilute solution of a carefully weighed, very 
nearly equivalent quantity of pure silver. The solution was allowed to 
stand for some days with occasional shaking, and then was tested in a 
nephelometer for excess of silver or bromide. If a deficiency of either 
was found, this deficiency was made up with very dilute standard silver 
or bromide solution. Then the solution was again tested, and the oper- 
ations were repeated if necessary. As soon as the exact end-point had 
been found, a moderate excess of silver nitrate was added, and eventually 
the silver bromide was collected and weighed. The usual correction 
for moisture retained by the dried silver bromide was determined and 
applied. 
